Question: Which of these numbers is prime? ${57,\ 69,\ 71,\ 77,\ 92}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 57 are 1, 3, 19, and 57. The factors of 69 are 1, 3, 23, and 69. The factors of 71 are 1 and 71. The factors of 77 are 1, 7, 11, and 77. The factors of 92 are 1, 2, 4, 23, 46, and 92. Thus, 71 is a prime number.